1. Field Of The Invention
The present invention relates generally to the field of surface chemical and topographical analysis. Specifically, the present invention includes a method and apparatus for analyzing the surface of a material using a combination of a scanning probe microscope (SPM) and laser induced breakdown spectroscopy (LIBS).
2. Description of the Prior Art
Imaging of chemically inhomogeneous surfaces is a common analytical task. Depending on the nature of the sample, the characteristic size of analyzed features, and the type of information required, various methods are employed. The majority of currently available commercial instrumentation utilizes particle beams originating from a remote source to probe the local chemical environment on the sample. Particles in the beam can be electrons (electron microscopy, Auger microprobes), ions (secondary ion mass spectrometry), X-ray photons (X-ray fluorescence) and other sources. A review of these well established techniques is described by Vickerman, xe2x80x9cSurface Analysis: the Principal Techniques,xe2x80x9d Chichester, N.Y., John Wiley (1997). Laser microanalysis methods are reviewed by Moenke-Blankenburg, xe2x80x9cLaser Microanalysis,xe2x80x9d Chichester, N.Y., John Wiley (1989).
The last decade has seen a rapid rise of scanning probe microscopy, SPM, as a prominent and versatile approach for surface studies. SPM instruments are differentiated from the beam-based ones by the fact that they use solid proximal probes for localized analysis. The most commonly used SPM methodology is atomic force microscopy, AFM. In its basic implementation, AFM provides topographical information with nanometer resolution. The most common modifications of AFM allow the magnetic, electrostatic, and specific chemical environment to be examined. See, Takano et.al., xe2x80x9cChemical and Biochemical Analysis Using Scanning Force Microscopy,xe2x80x9d Chemical Reviews 99 (1999) 2845-2890. All of the AFM methods described are indirect or measure a variable other than the measured parameter, but which has some type of dependence on the parameter to be measured. However, there is no direct way today to perform general chemical analysis with AFM probes.
Near-field scanning optical microscopy, NSOM, is another variation of SPM where sharp tapered optical probes, such as fibers or micro pipettes, serve dual purposes, being proximal probes of sample topography, and providing the means for localized light delivery for optical studies with sub-wavelength spatial resolutions. Again, NSOM itself does not have a general chemical contrast capability. However, the capability to deliver light to localized area opens the way to a multitude of experiments that can be devised using different aspects of light interaction with the sample.
One approach in this family of light-based methods is laser induced breakdown spectroscopy, LIBS. LIBS is widely used to study elemental composition of samples by analyzing optical emissions from pulsed plasmas created by a focused laser beam. It was pioneered by Radziemski in the early eighties. See, Radziemski Anal. Chem., 55 (1983) 1246-2486. Other names which are sometimes used to describe essentially the same technique are laser induced plasma spectroscopy (LIPS) and laser spark spectroscopy (LASS). Song and co-authors have recently published a review of LIBS applications. See Song, et.al., Appl. Spec. Rev. 32 (1997) 183-235.
It is an object of the present invention to provide a method and apparatus for simultaneous topographical and chemical analysis with high spatial resolution.
Another object of the present invention is to provide a method and apparatus for chemical imaging which is easy and inexpensive to operate relative to the available instrumentation for chemical imaging.
Yet another object of the present invention is to provide a method and apparatus for chemical imaging capable of operation in ambient conditions as opposed to vacuum based analysis techniques.
Another object of the present invention is to provide a method and apparatus for chemical imaging which requires minimal to no sample preparation as opposed to extensive sample preparation routines in vacuum based chemical analysis methods.
The present invention involves the combination of scanning probe microscopy and laser induced breakdown spectroscopy to provide spatially resolved chemical analysis of the surface correlated with the surface topography. Topographical analysis is achieved by scanning a sharp probe across the sample at constant distance from the surface. Chemical analysis by the means of laser induced breakdown spectroscopy is achieved by delivering pulsed laser radiation to the sample surface through the same sharp probe, and consequent collection and analysis of emission spectra from plasma generated on the sample by the laser radiation.
The method of the present invention includes performing topographical scanning simultaneously with or followed by chemical analysis via scanning laser induce breakdown spectroscopy.
The apparatus of the present invention includes a scanning mechanism with a means of bringing a probe in the vicinity of a sample, probe, pulsed laser source, collection optics interfaced to an optical spectrometer, and electronics controlling the instrument.
The probe may be a solid fiber or a hollow pipette having a hollow tip. The probe may be coated with an opaque coating on the sides and having transparent face provided that the coating is durable enough to withstand the laser pulse delivered through the probe. If such coated probes are used then the operation of the instrument is similar to that of a near-field scanning probe microscope.
For uncoated probes, it is possible to create chemical sampling spot size on the order of the wavelength of laser radiation used for plasma generation. This determines spatial resolution of chemical imaging. For coated probes the analytical spot size may be smaller if the analysis is performed in the near field, i.e. at probe-sample separation smaller than the aperture of the transparent face of the probe. In this case the spatial resolution of chemical imaging will be determined by the size of the aperture, which can be several times smaller than the wavelength of laser light used for plasma generation. Unlike near field spectroscopy the preferred embodiment contemplates an uncoated probe so that light from all or most of the plasma plume can be received by the probe in those embodiments where the probe is also used as the receiving device. The emitted light from the plasma plume is characteristic of the material at the target site, which is spatially well defined. Therefore, unlike near field spectroscopy sensed light can be collected from the entire field of view without losing any spatial resolution of the chemical analysis.
This analytical method is very attractive because of its simplicity, speed, affordability with virtually no requirement for sample preparation if the study is conducted under ambient conditions. Combination of LIBS and scanning probe microscopy delivers a simple and elegant way to achieve chemical contrast to complement topographical studies performed by SPM.
More specifically, the invention comprises an apparatus for performing chemical and topographical analysis of a sample. The apparatus comprises a probe proximal to the sample, and a scanner coupled to the sample or probe for scanning the probe relative to the sample. A pulsed laser is optically coupled to the probe. A light collector receives light from the sample. An optical spectrometer is optically coupled to the light collector. A controller coupled to the scanner, laser and spectrometer controls the operation of the scanner, laser and spectrometer in a correlated, coordinated or synchronized fashion. The probe and scanner are used for topographical profiling the sample. The probe also is used for laser radiation delivery to the sample for generating a plasma plume from the sample. Optical emission from the plasma plume is collected by the light collector and delivered to the optical spectrometer. Analysis of emission spectrum by the optical spectrometer allows for identification of chemical composition of the sample at user selected sites.
In one embodiment the probe comprises a tapered end of an optical fiber. An opposing end of the optical fiber is coupled to the pulsed laser. In another embodiment the probe comprises a drawn microcapillary having a sub-micron size diameter and a distal end. The laser light is coupled into the probe by focusing the laser beam onto an optical fiber disposed in the distal end of the capillary. In this case the probe comprises an optical fiber and a drawn microcapillary having a sub-micron size diameter and a distal end. The laser light is coupled into the probe by coupling the light into the optical fiber. The optical fiber is disposed into the distal end of the capillary.
The scanner is a linearized scanner providing precise relative positioning of the probe and sample within a range of 1000xc3x971000 micrometers. In this manner positioning of the probe for topological measurement and for chemical analysis at a selected site can be accurately made at different times.
In one embodiment the collector is comprised of a lens which collects the emitted light from the plasma plume created by the laser pulse. The emitted light then is delivered to the optical spectrometer. The apparatus may further comprise a mirror and the emitted light is delivered to the optical spectrometer by means of the mirror.
Alternatively, the apparatus further comprises a lens and an optic fiber. The emitted light is delivered to the optical spectrometer by means of direct imaging through a lens and optic fiber.
Still further apparatus further comprises an optical fiber and the emitted light is delivered to the optical spectrometer by means of the optical fiber.
In one embodiment the probe comprises a probe tip and an optical delivery path coupling the laser to the probe tip. The emitted light is collected by the same probe tip used for delivery of the laser pulse. The emitted light enters the probe tip and propagates along the optical delivery path in a direction opposite to delivery of light to the probe tip from the laser. The emitted light is delivered to the spectrometer.
The emitted light may be delivered to the spectrometer by means of a free space beamsplitter in the optical delivery path, or by means of a directional fiber coupler in the optical delivery path.
The invention is also characterized as a method for analyzing a material content and topography of a sample comprising the steps of performing topographical analysis of the sample by bringing a probe into a distance feedback relationship with the sample. The probe scans across the sample while maintaining constant separation between the probe and the sample. A scanned topological site on the sample is selected. Laser pulses are emitted from a pulsed laser. The laser pulses are coupled into an optical fiber. The laser pulses are delivered to the scanned topological site on the sample by means of the probe. A plasma is generated at the scanned topological site. A spectrum of optical emission from the plasma is measured. Specific chemical constituents are detected by analyzing line features of the collected spectrum.
The step of generating a plasma from the scanned topological site generates the plasma from the scanned topological site which is in the range of approximately 10 nm-2 xcexcm diameter. The step of emitting laser pulses from a pulsed laser emits pulses with a pulse length from about 1 attosecond to about 1000 femtoseconds in duration.
The step of scanning the probe across the sample is in the form of a raster of pixels. A chemical analysis comprises generating a plasma from the scanned topological site, measuring a spectrum of optical emission from the plasma, and detecting specific chemical constituents by analyzing line features of the collected spectrum is performed in each pixel of the raster. The chemical composition of the sample is recorded for each pixel of the raster and a chemical map of the sample is produced. The measurement of the spectrum of optical emission from the plasma is performed after or acquired with a variable time delay of 100 ns-5 microseconds after delivering said laser pulses.
Although the method have been grammatically described above for the sake of ease in terms of steps it is to be expressly understood that the claimed invention is not limited by the xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d restrictions of 35 USC 112. The invention having been briefly summarized, can now be better visualized by turning to the following drawings wherein like elements are reference by like numerals.